feat: Make the bigquery sample agent run with ADC out-of-the-box

Right now the bigquery sample agent is configured to run with OAuth, which requires some set up. This change makes it more readily usable, both locally and in AgentEngine, as Application Default Credentials (ADC) is easier to set up, and often local and AgentEngine environment already have it set up.

PiperOrigin-RevId: 808315879

Author: google-genai-bot

contributing/samples/bigquery/agent.py

@@ -22,8 +22,11 @@
 from google.adk.tools.bigquery.config import WriteMode
 import google.auth
 
-# Define an appropriate credential type
-CREDENTIALS_TYPE = AuthCredentialTypes.OAUTH2
+# Define the desired credential type.
+# By default use Application Default Credentials (ADC) from the local
+# environment, which can be set up by following
+# https://cloud.google.com/docs/authentication/provide-credentials-adc.
+CREDENTIALS_TYPE = None
 
 # Define an appropriate application name
 BIGQUERY_AGENT_NAME = "adk_sample_bigquery_agent"